oil change

[mfp4073]

My car is due an oil change next week, it was changed 12 months ago and has only covered 5ooo miles.
The oil in the engine is Royal Purple, and it still looks OK, i.e not black.
So the question is how long can you leave the oil before it degrades too much?
The reason I ask is that great claims are made about oil these days,so why can't you get a couple of years of normal driving out of it?

[mfp4073]

If Royal Purple is the same as any other oil, how can it perform better in test's and why don't other oil manufacturer's challenge Royal Purples claims?
As for oil consumption my engine has never needed a top up between servicing, I thought only tuned motors used extra oil.